[CarlJF]

Recently, with the announcement of the new Canon and Nikon latest DSLR, I hear more and more about f8 AF points. A Canon user even asked me recently how many f8 AF points do the K-1 have. No idea and, honestly, I don't care either.

So, I'm just wondering what is all this hype about f8 AF points, and why should I care about it ? To my understanding, these AF points should not be very accurate without any advantages over the usual 5.6 points, not even talking about f2.8 ones. Why would someone brag about them in a positive way about these AF points ? Am I missing something ? I'm more seeing these as a disadvantage and don't see why I would wish a lot of innacurate AF points...

[normhead]

I'm guessing with ƒ8 AF points, my A-400 with F-1.7x AF converter= ƒ9.3 would have a chance of focusing accurately. Currently, it doesn't.

[Venom3300]

Mainly for us long lens shooters that have a 1xx-4xx or 5xx type of lens or a 300 f4 that we want to put teleconverters on and retain autofocus.

[Nass]

Autofocus needs a certain amount of light to work well. Autofocus doesn't work as well at f/8 as it does at f/2.8. So if you have long lenses (ie say 500mm+) and you need to use f/8 or 'higher' (ie f/11 etc), then having more focus points that will work at f/8 is good. It's a big deal for birders, sport.

[CarlJF]

Thanks guys!

So, if someone isn't using a TC or some exotic small aperture lens, these f8 points don't offer any practical advantages over the more common 5.6 and 2.8 points ?

@Nass: I've just noticed that your the guy behind the extreme macro website. It's really a great website that has provided a lot of useful information when I made my first steps in macro photography last summer. It has really help me to rapidly get some decent results, by avoiding the most common pitfalls and be on the right track from the start. Things will surely be even better next summer!

[bertwert]

Thanks guys!

So, if someone isn't using a TC or some exotic small aperture lens, these f8 points don't offer any practical advantages over the more common 5.6 and 2.8 points ?

Pretty much.

[boriscleto]

With an F/8 sensor line you can autofocus with a F/5.6 lens & 1.4x TC. The trade off is less accuracy than say a F/2.8 sensor line.

Ricoh chooses to market it's 3 F/2.8 sensors (F2.8 luminous flux compatible AF) and emphasize the accuracy in low light (-3EV low-luminance AF).

[stevebrot]

The f/5.6 and f/2.8 (actually hybrid f/2.8/f/5.6) sensor prisms "black out" at apertures approaching f/8 and narrower with the result that focus is difficult or impossible to attain.* It is not an issue of accuracy or precision. The system simply fails at narrower apertures. I was not aware that any maker was tuning their sensors to extend their range down.

Edit: I did the Google work and found this market bit at Nikon that explains their support down to f/8 on the D800...

http://imaging.nikon.com/lineup/dslr/d800/features03.htm

Steve

* This problem is similar to split image and microprism "black out" on focus screens having those features.

---

Last edited by stevebrot; 03-09-2016 at 09:18 AM.

[derekkite]

It is a different way of stating -3ev or -4ev focussing. Essentially translating the technical term into something we can understand. I can focus a lens that is f8. A friend who shoots nikon has a 300 f4 and uses a 2x teleconverter. F8, and he has trouble getting focus in all but the brightest light.

If you have an f2.8 lens, you see the same effect in very low light conditions.

[Nass]

@Nass: I've just noticed that your the guy behind the extreme macro website. It's really a great website that has provided a lot of useful information when I made my first steps in macro photography last summer. It has really help me to rapidly get some decent results, by avoiding the most common pitfalls and be on the right track from the start. Things will surely be even better next summer!

Yep... that's me. It's my pleasure, delighted that it's been useful to you!

[CarlJF]

Edit: I did the Google work and found this market bit at Nikon that explains their support down to f/8 on the D800...

Nikon | Imaging Products | Still image shooting functions - Nikon D800/D800E

Yeah, although I write Canon user above, it was a Nikon one... It seems the D750 also have some f8 points and, obviously, the newest D500 and D5 have a lot. Although, this was what I understand from him. I guess he was a TC user since he talked about these points as the next best thing since slice bread. Since I'm not using a TC, I didn't see why someone would need or wish for them...

[BrianR]

Here are some diagrams showing how the AF modules gather light: Understanding Autofocus, Clarkvision.com. Some more general info here, including some photos of an actual AF-module (Canon-centric): Canon EOS DSLR Autofocus Explained

Somewhere there is a good Canon marketing sheet that explained the accuracy the different AF sensors were aiming for (within some specified fraction of a DoF), I can't find it now though

[stevebrot]

It is a different way of stating -3ev or -4ev focussing.

Not quite. It is a matter of physics. The sensor "blacks out"* at narrower aperture regardless of light level. That being said, the effect is more severe when light is precious. I used to have a link to a good PDF that explained both the wide and narrow aperture limits for PDAF, but can't find it now


Steve

* Not really a good way to explain what happens except that it has the same problem as prism blackout with a split-image focus screen.

[monochrome]

That's one of the reasons high-end professional film bodies e.g. LX, F2 / F3, etc. had interchangeable focusing screens. And why I have 2 KX'es and three K2's, with different semi-permanently installed (OEM) optional focusing screens.

[osv]

Recently, with the announcement of the new Canon and Nikon latest DSLR, I hear more and more about f8 AF points. A Canon user even asked me recently how many f8 AF points do the K-1 have. No idea and, honestly, I don't care either.

So, I'm just wondering what is all this hype about f8 AF points, and why should I care about it ? To my understanding, these AF points should not be very accurate without any advantages over the usual 5.6 points, not even talking about f2.8 ones. Why would someone brag about them in a positive way about these AF points ? Am I missing something ? I'm more seeing these as a disadvantage and don't see why I would wish a lot of innacurate AF points...

https://www.learn.usa.canon.com/resources/articles/2016/eos1dx_mkii/eos1dx_markii_f8_af.shtml

i think that f/8 is the break point, because you lose pdaf capability on all cameras(?) when the effective aperture is closed down more than that.

---------- Post added 03-09-16 at 11:25 AM ----------

Not quite. It is a matter of physics. The sensor "blacks out"* at narrower aperture regardless of light level. That being said, the effect is more severe when light is precious. I used to have a link to a good PDF that explained both the wide and narrow aperture limits for PDAF, but can't find it now

http://dougkerr.net/Pumpkin/articles/Split_Prism.pdf ???